In a number of processes, such as glass melting, in the metallurgical industry etc., the thermal energy required is obtained by burning gaseous products or hydrocarbons particularly natural gas, that can be easily converted into gaseous products. The combustion products (flue gas) released in such processes still contain a substantial amount of thermal energy in the form of sensible heat.
It is known that a part of this heat can be recovered by using the flue gas for preheating the air required for the combustion process. For this purpose use is often made of a metal radiation recuperator (a heat exchanger in which heat is transferred by radiation). The degree in which heat can be recovered, however, is highly limited by the temperature maximally permitted by the metal of the recuperator. In practice, this means that the air required for the combustion cannot be preheated beyond a temperature of about 800.degree. C. The temperature of the flue gas to be discharged to the chimney is still about 700.degree. C.
In order to make better use of the residual heat of the flue gases, it has been proposed that the gas can be used to thermally convert a methane-containing (natural) gas mixture with steam. In a so-called thermochemical recuperator (reformer) heat is transferred from the flue gas to a reacting natural gas-steam mixture which is passed over a steam reforming catalyst at high temperature and is converted into a mixture of hydrogen, carbon monoxide and carbon dioxide. With this reactor much heat is absorbed which is released again in the combustion of the resulting gas mixture (compare "The Thermochemical Recuperator System, Advanced Heat Recovery" by Donald K. Fleming and Mark J. Khinkis, paper presented by the 12th Energy Technology Conference and Exposition, Washington, D.C., Mar. 25-27, 1985).
The heat of the flue gases leaving the reformer is then transferred, optionally, after an intermediate stage in which the gas/steam mixture is preheated, to a steam boiler in which the steam is generated for the reforming process.
Although the use of the recuperator/reformer combination may in principle lead to a substantial increase in thermal efficiency, its practical realization has not been possible because of the incontrollability of the process in its non-stationary phase. This also applies when a part of the flue gas is passed from the furnace to the chimney either directly or via the recuperator, quite apart from the accompanying economic losses.
The major causes of the above are that during the starting-up phase of the combustion furnace and therefore previous to the equilibrium or stationary phase in which relatively large amounts of heat are dissipated in the process system the heat content and the temperature of the flue gases are so high that the flue gases cannot be passed through the reformer and preheaters. During the starting-up phase the reformer is in fact not or still insufficiently cooled by the endothermic reforming reactions, so that it would be damaged by the occurring high temperature. Similar problems occur when the phases of the process which take place after the recuperator must be discontinued, e.g., for replacement of the catalyst in the reformer or because of other failures.